(1) Field of the Invention
The present invention relates to a method according to the preamble of claim 1 to demodulate a quadrature amplitude- or phase-modulated i.e. phase shift keyed signal as well as a corresponding circuit arrangement according to the preamble of claim 8.
(2) Description of Related Art
The present broadband cable network, which serves to transmit analog TV signals, will in future also be used to transmit digital TV channels. Furthermore it will also serve as a transmission medium for interactive services, such as in particular internet applications. This demands that digital data must be transmitted via the broadband cable network instead of analog TV signals. In addition for interactive services a feedback channel must be made available, by means of which digital data can be sent back by the individual subscriber.
According to the current state of standardization for digital broadband cable networks for the outward channel, that is to say the transmission direction from the exchange or the so-called xe2x80x9chead endxe2x80x9d of the broadband cable network to a subscriber (so-called xe2x80x9cdownstreamxe2x80x9d direction), a transmission frequency range of between approx. 90 MHz and 800 MHz is specified, while for the feedback channel, that is to say the transmission direction from a subscriber to the exchange (so-called xe2x80x9cupstreamxe2x80x9d direction), a transmission frequency range of between approx. 5 MHz and 65 MHz is specified. In addition for data transmission the so-called quadrature amplitude modulation (QAM) is specified as type of modulation.
Quadrature amplitude modulation is an expansion of digital phase modulation or phase shift keying. Generally with phase shift keying the phase of the carrier signal is changed over between several discrete stages depending on the data symbol being transmitted. While in the case of pure phase shift keying a data symbol being sent is only transmitted in the form of carrier phase information modulated up to a carrier signal, with quadrature amplitude modulation N=1d(M) data bits are combined and transmitted in the form of carrier amplitude information and carrier phase information, where M describes the number of the different carrier states which can be transmitted.
The structure of a typical QAM modulator is illustrated in FIG. 4. The binary data being transmitted are firstly combined into groups with aid of an encoder 1 and in each case allocated to a value pair (ak, bk).
The number M of available value pairs exactly defines the type of modulation. The available value pairs can be represented in a 2-dimensional a/b graph, the so-called signal space. If only value pairs, which lie in the a/b-graph on the unit circle are used, pure phase shift keying is the case. If for example only the value pairs (1,1), (1,xe2x88x921), (xe2x88x921, 1) and (xe2x88x921, xe2x88x921) are used, the corresponding modulation is described as 4PSK modulation. If on the other hand the value pairs in the a/b-graph have different amplitude stages, quadrature amplitude modulation is the case.
As an example the signal space situation or the a/b-graph for 64 stage quadrature amplitude modulation is illustrated in FIG. 5A, while in FIG. 5B the signal space situation for 16 stage quadrature amplitude modulation is shown.
The values ak and bk of the value pair generated with the encoder 1 depending on the data being transmitted are then sampled in two separate signal paths with the aid of samplers 2 or 5, connected with a sampling frequency ft=1/T also described as symbol rate, to the inputs of two identical digital low-pass-filters 3 or 6, which have the transmission function g(t), and with the aid of multipliers or modulators 4 or 7 are multiplied by the carrier oscillations cos(2xc2x7xcfx80xc2x7f0xc2x7t) or sin(2xc2x7xcfx80xc2x7f0xc2x7t) orthogonal to each other. At the output end the two part signals resulting from this are added using an adder 8 and transmitted to a receiver in the form of a quadrature amplitude-modulated transmitted signal xs(t) via a transmission channel, in the case of a broadband cable network via a broadband-coaxial cable.
From the receiver the received signal must be demodulated coherently with the modulation shown in FIG. 4. Pure digital construction of the demodulator however is not possible on account of the relatively high transmission frequency range of approx. 90 MHz . . . 800 MHz in the case of downstream data transmission.
As is shown in FIG. 6 by way of a typical cable modem receiver, the quadrature amplitude-modulated received signal xE(t) in the receiver is therefore brought using a normal commercial tuner 9 or frequency mixer and a variable mix frequency fm1 into an intermediate frequency position, as is also normal for example in the case of analog TV reception. The intermediate frequency in Europe is normally 36 MHz and in the USA 44 MHz. Subsequently the received signal is fed via a bandpass filter 10 to a further frequency mixer 11, which shifts the received signal xE(t) lying initially in the intermediate frequency range mentioned dependent on a further (permanent) combination frequency fm2 into the frequency range of approx. 8 MHz . . . 15 MHz. Then the received signal xE(t) is fed to a digital demodulator 12 for demodulation. Equally it is conceivable that the received signal xE(t) is fed without further frequency shift to the digital demodulator 12, which is described as xe2x80x9cdirect conversionxe2x80x9d.
The structure of the actual digital demodulator 12 according to prior art is shown in FIG. 7.
As already mentioned, demodulation takes place coherently with the modulation shown in FIG. 4. The quadrature amplitude- or phase-modulated received signal xE(t) is first fed to an analog/digital-converter 13, which samples the received signal with a sampling frequency fs. Then the received signal digitalised in this way is fed to two signal paths, where it is multiplied with the aid of multipliers 14 or 17 by the two carrier oscillations   cos  ⁡      (          2      ·      π      ·                        f          0                          f          s                    ·      k        )  
or   cos  ⁡      (          2      ·      π      ·                        f          0                          f          s                    ·      k        )  
orthogonal to each other. Subsequently with the aid of digital filters 15 or 18, which have the transmission function h(t), low-pass filtering of the two received signal components takes place, in order to filter out or suppress spurious signal components in the case of the doubled carrier frequency as well as interfering components of adjacent frequency bands. With the aid of samplers 16 or 19, which work at least with a sampling frequency fT=1/T corresponding to the symbol rate, the components of the value pairs (ak, bk) corresponding to the particular sample value of the received signal xE(t) are retrieved and finally fed to an equalizer and decoder 20, which by way of a decoding ruling corresponding to the type of modulation used in each case calculates and emits the original binary data.
In the case of the demodulator arrangement shown in FIG. 7 however the relatively high signal processing complexity, which is necessary to produce the cosine- and sine values of the carrier oscillations and for multiplying by the carrier oscillations and for subsequent digital filtering, is disadvantageous, whereby this problem not only concerns quadrature amplitude-modulation, but all types of phase modulation or phase shift keying.
The present invention is therefore based on the objective of proposing a method and a corresponding circuit arrangement for demodulation of a quadrature amplitude- or phase-modulated signal, whereby the signal processing complexity is reduced.
This objective is achieved according to the invention by a method with the features of claim 1 or a circuit arrangement with the features of claim 6. The sub-claims in each case define advantageous and preferred embodiments of the present invention.
According to the invention a sampling frequency fs, which has the following relationship:       f    s    =            4      ·              f        0                    1      +              2        ·        λ            
with the carrier frequency fo of the quadrature amplitude- or phase-modulated signal is selected for demodulation. For multiplications to be performed in the cosine- or sine-signal path of the demodulator considerably less constructional complexity results since the particular sampling signal of the quadrature amplitude or phase-modulated signal only has to be multiplied by the value xe2x88x921, 0 or 1.
Further simplification results if the individual sample values of the quadrature amplitude- or phase-modulated signal are split with the aid of corresponding switchover directly into the two signal paths, whereby the switchover is performed according to the sampling frequency fs. The sampling rate of the two signal paths can therefore be halved, related to the sampling frequency of the analog/digital-converter at the input end. Before low-pass filtering, the sample values in the two signal paths must be multiplied alternately by +1 or xe2x88x921. The sampling frequency has a major influence on complexity of construction. The higher the sampling frequency, the greater the computing complexity necessary for demodulation.
As a result of the reduction described above of the sampling rate in the two signal paths complexity of construction can therefore be reduced.
The function of the analog/digital-converter at the input end can be split into two analog/digital-converter-stages, whereby the first stage only produce the high value bits of the individual sample values, while the second stage, which is designed in each case using a analog/digital-converter in each of the two signal paths, which generate the low value bits of the sample values.
The invention can be used in a receiver with xe2x80x9cdirect conversionxe2x80x9d or xe2x80x9cdirect first intermediate samplingxe2x80x9d as well as in a receiver with a second analog mixing stage (see FIG. 6).
The invention for example is suitable for use in a digital demodulator for QAM-signals, as for example it can find application in a digital receiver for broadband cable networks. The invention however is not limited to this field of use, but generally can be employed to demodulate quadrature amplitude- or phase-modulated i.e. phase-shift keyed signals.